The present invention relates to a device for discharging dust from a dry dust collector of a blast furnace.
The object of a dry dust collector of a blast furnace (as e.g. a dust-catcher, a cyclone or a filter) is to remove in a dry state as much as possible of the dust with which the blast furnace gas is heavily laden, before the blast furnace gas passes into succeeding wet-cleaning units. Such dry dust collectors generally have a cone bottom in which the blast furnace dust accumulates and from where it must periodically be evacuated.
Known devices for discharging the dust from the cone bottom of a dry dust collector in a blast furnace plant comprise a fully enclosed mechanical conveyor, normally a conveyor including propelled paddles for pushing the dust through a closed duct. The conveyor collects the blast furnace dust at a dust discharge opening in the cone bottom of the dry dust collector to drop it e.g. into a railroad car. Water is injected into the conveyor so as to slightly moisten the blast furnace dust, thus preventing the dust from blowing around too much as it drops out of the conveyor.
At least one isolating valve is mounted between the conveyor and the dust discharge opening. This isolating valve is used to interrupt dust discharge from the dry dust collector if a new rail road car has to be placed under the outlet of the conveyor or if the dust level in the cone bottom of the dry dust collector has fallen below a certain level. Maintaining a minimum dust level in the cone bottom of the dry dust collector indeed warrants that pressurised gas does not blow through the conveyor when the isolating valves are open.
It is also known to provide a lock with a gastight outlet and inlet valve between the screw conveyor and the dust discharge opening. The gastight dust discharge valve of the look is closed when dust is discharged from the cone bottom of the dry dust collector into the lock. The gastight inlet valve of the lock is closed when dust is discharged from the lock Into the screw conveyor.
Known devices for discharging blast furnace dust comprise isolating valves that have either a plate shaped or a convex cone shaped closing body mounted on an arm that is articulated laterally of the dust discharge opening, so that the closing body can be pivoted around a horizontal axis between a closed position and an open position, wherein it is located completely outside of the dust flow. These valve types are used because they are rather insensitive to clogging and abrasion.
A major problem with known devices for discharging blast furnace dust is that the discharge flow rate of the dust from the cone bottom of the dry dust collector or from the lock is not at all constant This is inter alia due to the fact that the dust can be very fluid at one moment, but can also bake together one moment later. It follows that the operating conditions of the conveyor downstream of the discharge valve are subjected to violent changes. At one moment the conveyor may be running idle and just a moment later there may be a complete clogging in the conveyor.
It will be noted in this context that the standard solution for warranting a more or less uniform discharging of dustlike materials from a hopper is a cellular wheel. This solution is however not at all suited for discharging blast furnace dust from a dry dust collector because the blast furnace dust easily clogs the wheel cells and is also too abrasive for the wheel. Indeed, after a relatively short operating time, the lock edges of the cellular wheel are heavily worn out, so that blast furnace dust simply runs through the stopped cellular wheel.
JP 59-185711 discloses a method for automatically controlling the discharging of blast furnace dust from a dry dust collector. The discharging device disclosed in this document comprises three discharge valves in series, which are successively closed by a microcomputer. A mechanical conveying system, which comprises a screw conveyor and an enclosed belt conveyor in series, is located downstream of the dust discharge valves for providing mechanized transport of the dust discharged through the discharge valves. An oxygen sensor measures the oxygen content in the belt Conveyor. When the sensor detects a sharp decrease in the oxygen content, the discharge valves are closed by the microcomputer. According to JP 59-185711 a sharp decrease in the oxygen content is an indication of a blast furnace gas break-through and an automatic closing of the discharge valves in case of such a gas break-through helps to prevent environmental pollution by black smoke. It will be noted that JP 59-185711 does not concern itself with an improvement of the operating conditions of the mechanical conveying system.
The technical problem underlying the present invention is to provide a simple device for discharging blast furnace dust from a dry dust collector with improved discharging conditions.
The present invention concerns a device for discharging dust from a dry dust collector of a blast furnace comprising a dust discharge valve located downstream of a dust discharge opening of the dry dust collector and a fully enclosed dust conveying system located downstream of the dust discharge valve. In accordance with an important aspect of the present invention this device comprises a control system that is designed so as to control the opening of the dust discharge valve in function of the residual conveying capacity of the dust conveying system. In other words, the control system closes the discharge valve before the conveying system risks to clog and opens the discharge valve if the conveying load drops. It follows that the average conveying capacity, i.e. average discharge capacity, is substantially increased, whereas the risk of clogging is nevertheless reduced. As a result, breakdowns of the conveying system are less frequent and less overhauling operations are necessary. It will further be appreciated that in a conveying system that is more equally loaded, a gas break through is less probable. Consequently, with a device in accordance with the invention, it is generally not necessary to provide a lock with a gastight outlet and inlet valve between the dust discharge opening and the conveyor.
The conveying system of the device may comprise a fully enclosed mechanical conveyor, wherein the control system is designed so as to control the opening of the dust discharge valve in function of the power absorbed by the mechanical conveyor.
In a preferred embodiment of the device, the fully enclosed conveying system comprises a pneumatic conveying system, wherein the control system is designed so as to control the opening of the dust discharge valve in function of the pressure in the pneumatic conveying system. This device allows to efficiently discharge the blast furnace dust from the dry dust collector in a completely closed circuit. A preferred embodiment of such a pneumatic conveying system for the blast furnace dust comprises: a dust storage hopper located near the blast furnace; a pneumatic conveying conduit connected between the dust discharge valve the dust storage hopper, for transporting the discharged dust from the dry dust collector to the dust storage hopper; a fluidizing hopper connected to the dust storage hopper; and injection means for injecting the fluidized dust into the blast furnace.
It will be appreciated that the present invention also provides a dust discharge valve that is excellently suited for providing the required control function in the discharging device. This dust discharge valve includes: a housing with an inlet opening bounded by a concave annular surface that lies on a first fictive cylinder having a horizontal central axis; and a closing body mounted in the housing so that it can be pivoted around the horizontal central axis between a closed position and an open position. The closing body of this valve has a convex cylindrical closing surface that lies on a second fictive cylinder, which has a diameter that is slightly smaller than the diameter of the first fictive cylinder. In order to prevent that this valve is damaged or blocked by bigger nodules of agglomerated dust during the closing operation, its closing body is advantageously provided with a cutting edge that sweeps across the inlet opening when the closing body is pivoted between the closed position and the open position. Such a cutting edge is capable of shearing even very hard nodules of agglomerated dust when the valve closes. It will be noted that the cutting edge is advantageously given a concave form, so that shearing takes place mainly during the end phase of the closing movement. If the concave cutting edge is moreover substantially symmetric with regard to a center plane of the closing body, it is warranted that the closing body is more or less symmetrically charged during the shearing operation.
In a preferred embodiment of the discharge valve, the closing body includes a cylindrical closing plate with two lateral flanges, each of the flanges supporting a lateral journal. The housing of this valve includes a vertical dust passage channel below its inlet opening and a bearing located on either side of the dust passage channel for supporting each of the two journals of the closing body, so that the latter can be pivoted around the aforementioned horizontal central axis. It will be appreciated that the vertical dust passage channel is completely free when the closing body is in its open position. In other words, no component of the valve is located in the highly abrasive dust stream in the vertical dust passage.
In a preferred embodiment the closing includes a cylindrical closing plate with two lateral flanges, wherein the closing plate and the lateral flanges are made in one piece of metal carbide. Secured to each of these metal carbide flanges is a mating steel flange that supports a steel journal. It will be appreciated that this design makes it possible to obtain a metal carbide closing body with two perfectly aligned journals.
The housing of the discharge valve includes advantageously an inflatable seal, which is mounted around the concave annular surface so that it is pressed against the closing surface in the closed position when it is inflated, but spaced therefrom when it is deflated. This embodiment of the discharge valve warrants a gastight closure. It will be appreciated that the sealing element has a considerable service life, because it is exposed neither to contact with the dust stream, nor to contact with the pivoting closing surface.
The housing of the valve may furthermore include an outlet opening bounded by a ring made of metal carbide, wherein the ring is mounted in a ground plate below the closing body, so as to define a retaining shoulder for retaining a dust cushion on the ground plate around the outlet opening. This embodiment of the discharge valve distinguishes itself by an excellent wear resistance of the valve housing at the level of its outlet opening.